Interferometers, such as sonars, can be used to detect and locate objects. An active sonar projects a beam-like sound wave signal into the water, and an object that is in the path of the signal reflects a portion of the signal back to the sonar. The signal reflected back to the sonar is detected by the sonar and utilized to locate the object in the water.
FIG. 1 diagrammatically illustrates a conventional active sonar system 10 that is being used to locate an object 12 positioned below the surface of a body of water 14. The conventional sonar system 10 includes a projector 16 that projects a sound wave signal s(t) into the water 14. Portions of the signal projected from the projector 16 are reflected from the object 12 to a lower receiver 18 and an upper receiver 20. The lower receiver 18 and the upper receiver 20 are separated by a vertical separation distance "d". A direction of return arrow 22 illustrates the direction of travel of the reflected portions of the signal (that is, the echoes) that are received by the upper receiver 20. A vertical arrival angle .theta. that partially defines the position of the object 12 is estimated by measuring the difference in the time of arrival of the echoes at the lower receiver 18 and the upper receiver 20. The vertical arrival angle .theta. is the angle of intersection between a line perpendicular to the direction of the return arrow 22 and the vertical plane of the receivers 18 and 20. The vertical arrival angle .theta. is solved from: EQU D=d sin(.theta.)/c
Inputs for solving for the vertical arrival angle .theta. from the above equation consist of:
replica of output from the projector 16: EQU r.sub.1 (t)=s(t)+n.sub.1 (t) PA0 echo at the lower receiver 18: EQU r.sub.2 (t)=As(t-.tau.)+n.sub.2 (t) PA0 echo at the upper receiver 20: EQU r.sub.3 (t)=As(t-.tau.-D)+n.sub.3 (t)
In the immediately preceding equations, n.sub.1 (t) represents noise and is typically zero for a noise-free replica. In addition, n.sub.2 (t) and n.sub.3 (t) are uncorrelated zero mean white Gaussian noise waveforms. The signal s(t), which is projected, or transmitted, from the projector 16, is a pulse of length T seconds. The term "A" is the amplitude of the signal s(t). The delay ".tau." is the delay between transmission of the signal s(t) and reception of the corresponding echo at the lower receiver 18. The delay "D" is the delay between the reception of the echo at the lower receiver 18 and the upper receiver 20. The term "c" is a constant representing the speed of the projected signal s(t) through the respective medium, which in accordance with the preferred embodiment of the present invention is the speed of sound in water.
The conventional sonar system 10 requires both of the receivers 18 and 20 in order to determine the vertical arrival angle .theta.. It is typical for each of the receivers 18 and 20 to include an array of many receiver elements, such as forty-eight receiver elements, and for each of the receiver elements to require its own analog conditioning electronics and analog-to-digital converter. That is, each of the receivers 18 and 20 includes an array of tens of hydrophones, and each hydrophone requires a separate electronics assembly. Further, digital processing circuitry is required for each of the receiver elements. Additionally, it is typical for the projector 16 to include an array of many projector elements, such as sixteen projector elements and for each projector element to require its own analog conditioning electronics and digital-to-analog converter. That is, the projector 16 includes an array of transducers, and each transducer requires a separate electronics assembly. Further, digital processing circuitry is required to present the desired signal to each of the projector elements.
Conventional sonar systems 10 are often deployed on watercraft, such as unmanned undersea vehicles, and in some cases the resources associated with the watercraft, such as space, power, weight and processing capability, are limited. Thus, the inclusion of many receiver elements, and their associated electronics assemblies and digital processing, can be prohibitive. In addition, the cost of the electronics for all of the parallel processing channels can be substantial.
It is also known for an interferometer to have two projectors and one receiver. For example, see H. Messer, G. Singal, and L. Bialy, On the Achievable DF Accuracy of Two Kinds of Active Interferometers, IEEE Transactions on Aerospace and Electronic Systems, Vol. 32, No. 3, July 1996, pp. 1158-1164, which is incorporated herein by reference.
The use of a sonar system having two separate projector arrays, each having many projector elements and their associated electronics assemblies and digital processing, and one receiver array having many receiver elements and their associated electronics assemblies and digital processing could be prohibitive in situations where space, power, weight and processing resources are limited. And again, the cost of the electronics for all of the parallel processing channels can be substantial. In addition, when using two separate projector arrays with many projector elements, each projector array may interfere with the other projector array by at least partially physically blocking the signals of the other projector array.